Australian researchers have reported the development of a new molecular coupling tool combining green light and pH triggers.
Simply by varying pH levels between acid and alkaline, they can switch the photochemical reaction on and off, opening up a range of applications in medicine and manufacturing.
The green light, they say, is the longest wavelength of light (up to 500 nanometres) employed to date to control a catalyst-free photochemical bond-forming reaction.
The work by a team from Queensland University of Technology was led by Kubra Kalayc. The findings are published in the journal Nature Communications.
“Our photochemical innovation is another example of what is called red-shifting – moving through the colours of light in the spectrum, from blue to green towards red, to light that has longer wavelengths,” says co-author Christopher Barner-Kowollik.
“In the past, most of these types of photochemical reactions were triggered by harsh UV [ultra-violet] light. But that prevents applications in a biological context because UV light has so much energy it kills cells.”
Longer wavelengths are better in principle, and the radiation is less harmful, but they also present challenges because there is less energy to drive the chemical reaction.
“Adding an additional stimulus with the green light, such as we have with varying the pH as a reversible on-off switch for the reaction, provides the opportunity for better regulation,” Barner-Kowollik says.
“This is especially important for drug delivery systems, where the drug needs to be released under a specific pH, as pH varies throughout the human body.”
Also important, he adds, is that it’s a catalyst-free reaction with no “helper” molecule; these often contain metal, which can leach out.
To test their approach, the researchers used it to produce a range of hydrogels. The green light allowed higher penetration depths, Kalayc says, resulting in thicker gels.
The process was non-toxic, and the cells cultured inside the gels remained viable for several days.
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